1. Field of the Invention
The present invention relates to a varifocal projection lens system for projecting an original image field, for example, an image produced by a liquid crystal display, onto a screen.
2. Description of the Related Art
In order to project an original image produced by, e.g., a liquid crystal display, onto a screen which is placed above or below the optical axis of a projection lens apparatus, it is known to use an apparatus for shifting the optical axis of the projection optical system up or down from the center of the original image without changing the directions of the optical axis, as shown in FIGS. 5(A) and 5(B). FIGS. 5(A) and 5(B) illustrate one known arrangement of an optical system where the optical axis is not shifted from the center of the original image 3 and where the optical axis is shifted therefrom, respectively. FIGS. 5(A) and 5(B) each further show a projection lens 1 and a condenser lens 10. The condenser lens 10 directs the largest-possible amount of light traveling in parallel with the optical axis from the original image 3 to the projection lens 1.
However, that known apparatus requires a substantially large condenser lens in order to prevent a vignetting (or eclipse) by the condenser lens. Another problem of that known apparatus is that the mechanism required for moving both the projection lens 1 and the condenser lens 10 without changing their relative positions is substantially large. Further, if the optical axis is shifted, the brightness of an edge portion of the image field is reduced.
Japanese Patent Application Laid-open No. 61-90584 illustrates a projection apparatus that employs a polarizing beam splitter and a prism. However, the polarizing beam splitter and the prism cause the apparatus to be large and heavy, thus impeding the portability thereof. Further, the machining and polishing required for the prism adds substantial processing hours and costs.